1. Field of the Invention
The present invention relates to a flow sensor suitable for the measurement of the very low flow rate of a gas etc. used in, for example, a semiconductor manufacturing apparatus.
2. Description of the Related Art
For example, as a flow sensor (flow rate measuring device) for detecting the flow rate of a fluid to be measured such as a gas used in a semiconductor manufacturing apparatus, a thermal flow sensor has been known which measures a very low flow rate by measuring the temperature difference of a fluid at a predetermined position by adding heat to the fluid (for example, refer to Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-168669 (pp. 5-6, FIG. 1), Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-325335 (pp. 6-7, FIG. 8), and Patent Document 3: Japanese Unexamined Patent Application Publication No. 2007-071687 (pp. 2-3, FIG. 7)). As described in Patent Document 3, the thermal flow sensor includes a sensor chip formed with a flow rate detecting part on the upper surface thereof and a glass chip, serving as a flow path forming member, bonded to the upper surface, on which the flow rate detecting part of the sensor chip is formed, of the sensor chip with frit glass or the like.
The sensor chip has a heater (Rh) that is formed with an electric insulating film layer on the upper surface of a silicon substrate and forms the flow rate detecting part therein, an upstream-side temperature measurement sensor (Ru), a downstream-side temperature measurement sensor (Rd), and an ambient temperature sensor (Rr), each of which is formed by a platinum thin film. The platinum thin film functions as a temperature measurement resistor because the resistance value thereof changes according to temperature. The heater (Rh) is arranged in the central part of the substrate, and on both sides thereof, the upstream-side temperature measurement sensor (Ru) and the downstream-side temperature measurement sensor (Rd), serving as temperature measurement sensors, are arranged. The ambient temperature sensor is arranged in the surrounding part of the silicon substrate.
The central part on the upper surface of the silicon substrate formed with the heater (Rh), the upstream-side temperature measurement sensor (Ru), and the downstream-side temperature measurement sensor (Rd) is formed into a concave part by removing the silicon in the lower part thereof by anisotropic etching, and the heater (Rh), the upstream-side temperature measurement sensor (Ru), and the downstream-side temperature measurement sensor (Rd) have a diaphragm construction thermally isolated from the silicon substrate.
The glass chip is formed with a flow path for a fluid flowing on the flow rate detecting part, a fluid inflow port for introducing the fluid to be measured into the flow path, and a fluid outflow port for delivering the fluid to be measured which is introduced into the flow path. These flow path, fluid inflow port, and fluid outflow port are formed by sandblasting or the like. The flow rate detecting part is arranged so as to be exposed into the flow path, and measures the flow rate of the fluid to be measured such as a gas flowing in the flow path by detecting the temperature difference of the heater (Rh) between the upstream-side temperature measurement sensor (Ru) and the downstream-side temperature measurement sensor (Rd).
FIGS. 1 and 2 show one example of a thermal flow sensor relating to the present invention. A thermal flow sensor 501 is formed by bonding a sensor chip 504, in which a flow rate detecting part 503 is formed on a silicon substrate 502, and a transparent glass chip 505 serving as a flow path forming member, which accommodates a flow rate detecting part (sensor part) 503 and is formed with a flow path (groove) 505a for a fluid flowing in the flow rate detecting part (sensor part) 503, to each other. The flow path 505a of the glass chip 505 is formed by sandblasting or the like.
By configuring the flow sensor 501 in this manner, in the inspection process after the manufacture of the flow sensor 501, whether or not an abnormality is present in the flow rate detecting part 503 and the flow path can be checked visually. Further, check can be made to see whether or not minute dust etc. mix in the fluid to be measured after the use and enter into the flow path 505a, or whether or not the dust etc. give any trouble to the flow rate detecting part 503.
FIG. 3 shows a manufacturing method for the flow sensor relating to the present invention. As shown in FIG. 3A, a plurality of sensor chips 602 are formed on the upper surface (top surface) of a silicon wafer 601, and the wafer 601 is cut by dicing, by which the plurality of sensor chips 602 are separated as shown in FIG. 3B. Then, as shown in FIG. 3C, a glass chip 604 formed with a flow path 603, a fluid inflow port 603a, and a fluid outflow port 603b is placed on each of the individually separated sensor chip 602, and is positioned so that a flow rate detecting part is exposed into the flow path 603, by which a flow sensor 605 is manufactured by bonding the glass chip 604 to the upper surface of the sensor chip 602 with a glass having a low melting point, for example, frit glass or the like. The glass chip 604 is also formed individually like the sensor chip 602 by dicing a wafer on which the plurality of glass chips 604 are formed.
First, a first problem to be solved in the present invention is explained. In the method for forming the flow path 505a of the glass chip 505 by sandblasting, it is difficult to uniformly fabricate the height h from an upper surface 504a of the sensor chip 504 to an inside upper surface 505b of the flow path 505a of the glass chip 505 (hereinafter referred to as a “sensor flow path height”) as shown in FIGS. 1 and 2, so that it is difficult to enhance the fabrication accuracy. Therefore, the cross-sectional area S (=h X w, w is the width of the flow path 505a) of the flow path 505a near the flow rate detecting part 503 is sometimes not as designed. Since the flow rate Q is determined by the cross-sectional area S of the sensor flow path, the flow rate property (flow rate curve) changes as the cross-sectional area S of the flow path 505a changes. Therefore, adjustment of large span is needed, and it is difficult to achieve the stability of quality because the individual difference between the flow sensors arises.
Also, if the flow path 505a of the glass chip 505 is fabricated by sandblasting, the surface of the inside upper surface 505b of the flow path 505a, which is a fabricated surface, becomes rough and has poor transparency. Therefore, to enhance the transparency, post-treatment is needed. Also, the pressure resistance of glass chip may decrease due to a minute flaw induced at the time of sandblasting.
Successively, a second problem to be solved in the present invention is explained. The flow rate detecting part of flow sensor is formed into a very thin and small shape to enable a very low flow rate to be measured with high sensitivity. Therefore, the flow rate detecting part is easy to break, and if dirt adheres to the flow rate detecting part, a change in heat balance between the upstream side and the downstream side of heater cannot be detected well.
Thereupon, when the wafer 601 on which the plurality of sensor chips 602 are formed is cut by dicing to separate the sensor chips 602 as shown in FIG. 3, a complicated manufacturing process is needed to protect the flow rate detecting part from the adhesion of swarf and to protect it from the shock of a liquid coolant used in dicing. As a result, the production efficiency of flow sensor decreases, and also the yield of product decreases.
Also, in order to lessen an influence of swarf and liquid coolant on the flow rate detecting part when the wafer 601 on which the plurality of sensor chips 602 are formed is cut, for example, the use of a laser dicing device or the like can be thought. However, the laser dicing device is very high in cost, and accordingly the manufacturing cost of flow sensor increases. Also, even in the case where the laser dicing device is used, in the process from the separation of the sensor chips 602 from the wafer 601 to the bonding of the glass chip to the sensor chip 602, dirt may adhere to the flow rate detecting part of the sensor chip 602.
Also, if the individually separated glass chip 604 is lappingly bonded onto the upper surface of the individually separated sensor chip 602 as shown in FIG. 3C, the external dimensions varies due to a positional shift at the time of bonding, and also it is difficult to flush a side surface 605a of the flow sensor 605, so that a level difference is produced. If such a level difference is present, the dimensional tolerance of chip external shape varies, and therefore the positioning accuracy at the time of packaging of flow sensor decreases.